I suggest you read the FAQ.
You can gain exp by simply voting on submissions on newgrounds. Simply use the voting panel to vote 0, 1, 2, 3, 4, or 5 on a flash. Voting is not the same reviewing, however, so make sure you're voting on submissions, if you want to increase your exp. Once you have voted on five portal submissions, simply click the large link that you are prompted that says:
"Click here to deposit your experience!!"
You can only gain a total of 10 exp points a day (EST), which are gained through the method just explained. The only other possible ways you could have gotten exp, would have been through making a profile (20 exp) and signing up for newgrounds related emails when making an account (5 exp per mailing list). Other than that, there are no other possible ways of gaining exp.
Gaining experience points allows you to do awesome things like level up, and you can find a list of all the level icons and the exp you need to get to the corresponding level here!